Larry,

Thanks for posting this - that's pretty creative troubleshooting! I've had this problem with my 300E for awhile - eventually R&R'd the tranny mount, center support, and had the driveshaft bearings replaced, which has helped a lot, but the vibration isn't totally gone. Might try this at some point, depending on whether the vibration eventually gets worse again.

Incidentally, when the driveline shop was checking the driveshaft, they also found a "detent," or some resistance, in the bearings too.